The invention relates to gas turbines, and more particularly to mounting a starter/generator (S/G) on a gearbox, or an accessory gearbox. The field of application of the invention is that of gas turbines for aeroengines for airplanes or helicopters, and also that of auxiliary power units (APUs).
In a gas turbine, certain pieces of equipment or “accessories” are driven by a mechanical transmission using mechanical power taken from a turbine shaft. The mechanical transmission includes a set of gearwheels housed in a casing and is referred to as a gearbox or as an accessory gearbox. Accessories include in particular various pumps for producing hydraulic energy, of for delivering fuel or lubricant, and also one or more electrical S/Gs.
While a gas turbine is in operation, the or each S/G acts as an electricity generator and produces electricity that powers one or more electricity distribution centers for the airplane or the helicopter and its engine(s).
When the gas turbine is stopped, an S/G can be used as a starter by being connected to an external power supply in order to set the gas turbine into operation by rotating the turbine shaft to which the gearbox is connected.
An S/G of known type comprises a main synchronous generator having a main rotor and a main stator, together with an exciter having a rotor with a secondary magnetic circuit and a stator with a primary magnetic circuit. The secondary circuit of the exciter powers the main rotor of the synchronous generator via a rectifier such as a rotary diode bridge. In electricity generation mode, the secondary circuit of the synchronous generator produces an alternating voltage as a result of the primary circuit being rotated under power from direct current (DC) delivered by the diode bridge of the exciter, the frequency of the alternating voltage that is produced varying as a function of the speed of rotation. In starting mode, the main rotor powered by the exciter and the main stator powered by an alternating voltage from an external source act like a synchronous motor.
Such a known S/G is a relatively bulky piece of equipment that is usually mounted in a special casing on one side of the gearbox and that is mechanically connected thereto. This leads to occupying a large amount of space and to presenting a significant weight that is cantilevered out and therefore requires attachment means of sufficient strength to enable it to be mounted on the gearbox.